Pump pot partially drains after shut-off

[Poconos]

OK, something I just don't understand. How can water drain out of a pump when it's below the pool water level? Are there ANY components, hoses etc, that are above the waterline? Air gets into a system by negative pressure. With everything below the waterline there should be no part of the system under a negative pressure. If everything is below the waterline then opening any part of the system will let water out, not air in.
Al

[AnnaK]

I don't understand it either, Al. No part of the system is above the water line.

When I checked the skimmer sock that day It was full of bracts and soggy pollen. Is it possible that it was so clogged that not enough water went through the skimmer into the pump? I've kept an eye on it these past 2 days and have changed skimmer socks twice daily, and the issue seems to have resolved itself. The pot is filled completely and has its normal small bubble directly in the center of the domed lid. I can't help but think the pump was starved for water when the sock was dirty.

[mas985]

I can't help but think the pump was starved for water when the sock was dirty.

When the sock is dirty it will create higher suction at the pump which in turn can draw air into the lines from small leaks. By morning, a lot of air can remain in the pump basket because there is not enough flow rate, due to the higher head loss of the dirty sock, to remove the air.

[AnnaK]

Thanks Mark. Looks like that's what may have been going on.

[Poconos]

I kind of figured there was more going on but never thought along the plugged suction path. Glad the problem resolved itself.
Al